Implement strStr().

Return the index of the First occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1: Input: haystack = "hello", needle = "ll" Output: 2  Example 2: Input: haystack = "aaaaa", needle = "bba" Output: -1

Clarification:
What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this PRoblem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

难度: easy

题目：
实现方法strstr()
返回首次needle出现在haystack中的索引，如果不存在则返回 -1.

Runtime: 7 ms, faster than 37.21% of Java online submissions for Implement strStr().
Memory usage: 26.9 MB, less than 32.93% of Java online submissions for Implement strStr().

class Solution {     public int strStr(String haystack, String needle) {         if (needle.length() > haystack.length()) {             return -1;         }                  for (int i = 0; i <= haystack.length() - needle.length(); i++) {             int j = 0;             for (; j < needle.length(); j++) {                 if (haystack.charAt(i + j) != needle.charAt(j)) {                     break;                 }             }             if (j == needle.length()) {                 return i;             }         }                  return -1;     } }